The problem

3x-(1+2x)=6 Please pretend the parenthesis are aboslute value bars

my solution

$\displaystyle -(1+2x)=-6+3x $divide both sides by -1

$\displaystyle (1+2x)=6-3x$

$\displaystyle 1+2x=6-3x$ or $\displaystyle 1+2x=-6-3x $

x= 1 or x=7, but only seven is valid in the original equation. Why?

Thank you in advance!